The present invention relates to plate-fin exchangers having textured surfaces and to methods for assembling such plate-fin exchangers. The plate-fin exchangers having fins with textured surfaces according to the present invention have particular application in cryogenic processes such as air separation, although these plate-fin exchangers also may be used in other heat and/or mass transfer processes.
Plate-fin exchangers are generally used for exchanging heat between process streams for the purpose of heating, cooling, boiling, evaporating, or condensing the streams. In this case they may be referred to more particularly as plate-fin heat exchangers. The process conditions in these heat exchangers may involve single phase or two phase heat transfer, wherein the fluid streams flow in a generally upward direction or in a generally downward direction (although the flows may also be in other directions). But in some cases the process streams include mixtures of components so that mass transfer separation also is carried out in addition to heat transfer. In the latter case, vapor and liquid flow in countercurrent directions within a stream passage and the heat/mass exchanger may be referred to as a dephlegmator.
It is known from the prior art that there are several ways to enhance the performance of heat exchangers. See, for example, D. A. Reay, xe2x80x9cHeat transfer enhancementxe2x80x94review of techniques and their possible impact on energy efficiency in the UK,xe2x80x9d Heat Recovery System and CHP vol. 11, No. 1, p. 1-40, 1991; Some of the techniques known in the prior art include:
the surfaces of some heat exchangers can be roughened to improve the heat transfer performance in single phase flow by promoting turbulence in the boundary layer;
the surfaces of some heat exchangers can be treated with special coatings or modified geometrically to create reentrant cavities which can improve the performance in nucleate boiling;
the surfaces of some heat exchangers can be treated or modified geometrically in order to alter wetting by liquids which can improve the performance by promoting drop-wise condensation or facilitating drainage of the condensate; and
while all of the above techniques are applicable to plate-fin heat exchangers, their performance is most readily improved by the use of perforated, serrated or wavy fins which increase the turbulence relative to plain fins.
However, as persons skilled in the art will recognize, each of the prior art techniques are limited in one or more ways. For example, the improvements obtainable may be limited to single flow applications, to a narrow range of flow and operating conditions, or to a single mode, such as condensation.
An example of the surfaces of a plate-fin heat exchanger being modified is disclosed in U.S. Pat. No. 4,434,842 (Gregory). In this heat exchanger, fins in the boiling regions are made of at least two layers, with at least one of the outer layers having a plurality of holes therein. The corrugated sheets of the fins are in close proximity one to the other such that nucleation of bubbles occurs between the sheets and the bubbles are released by the holes in the sheets.
Although Applicants are not aware of any prior art plate-fin heat exchangers in which the fins have a surface texture in the form of grooves or fluting (such as that used in the present invention), such surface texture has been used on other types of heat exchangers (e.g., shell and tube exchangers) to create or enhance turbulence and improve heat transfer. For example, see U.S. Pat. Nos. 4,434,842; 6,012,514; and 5,966,809. However, in addition to the fact that those patents do not pertain to plate-fin heat exchangers, the teachings of those patents are not pertinent to the teachings of the present invention.
In the field of contact processes which use structured packing, it is well known that surface texture in the form of fluting or grooves can improve mass transfer efficiency, as taught in U.S. Pat. No. 4,296,050. See also U.S. Pat. Nos. 5,730,000 and 5,876,638. These patents teach the use of a bidirectional surface texture in the form of fine grooves applied in patches on the surface of corrugated plates of a packing element such that the texture is substantially horizontal in some regions and substantially vertical in other regions. But this improvement is based on the experience in a specific operating mode, namely downwardly flowing liquid film undergoing mass transfer against vapor which flows upward in a direction countercurrent to the liquid flow. The present invention has a much broader scope and range of applications than that. Also, the overall geometry and flow characteristics within a plate-fin exchanger are very different from those of a structured packing even for generally similar operating modes.
It is desired to increase the efficiency and improve the performance of plate-fin exchangers.
It is further desired to improve the wetting characteristics of a downwardly flowing vapor-liquid stream within the passages of a plate-fin exchanger in order to improve the heat transfer efficiency.
It is still further desired to improve the flow characteristics of an upwardly flowing vapor-liquid stream within the passages of a plate-fin exchanger in order to improve the heat transfer efficiency.
It is still further desired to improve the turbulence characteristics of a single phase stream within the passages of a plate-fin exchanger in order to improve the heat transfer efficiency.
It is still further desired to improve the turbulence characteristics within the flow passages of a counter-current dephlegmator in order to improve the mass transfer efficiency relative to a conventional plate-fin exchanger employed under similar operating conditions.
It is still further desired to improve the wetting characteristics of a downwardly flowing vapor-liquid stream within the passages of a plate-fin exchanger such that the tendency to precipitate out any dissolved components is minimized.
It is still further desired to have a plate-fin exchanger or dephlegmator that shows high performance characteristics for cryogenic applications, such as those used in air separation, and for other heat and/or mass transfer applications.
It is still further desired to have a plate-fin exchanger which overcomes many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.
It is still further desired to have a more efficient air separation process utilizing a plate-fin exchanger or downflow reboiler which is more compact and/or more efficient than the prior art.
It is still further desired to have a plate-fin exchanger design which minimizes the size, weight and/or cost of downflow reboilers, which would result in an air separation process more efficient and/or less expensive per unit quantity of product produced.
It also is further desired to have a method for assembling a plate-fin exchanger or a downflow reboiler which uses fins having a surface texture thereon which affords better performance than the prior art, and which also overcomes many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.
The present invention is a plate-fin exchanger having textured surfaces. The invention also provides a method for assembling such a plate-fin exchanger, and a method for improving the performance of a plate-fin exchanger. The xe2x80x9ctextured surfacexe2x80x9d used in the present invention to obtain a xe2x80x9csurface texturexe2x80x9d is in the form of grooves or fluting formed on or applied to the surface of the fin material used in the plate-fin exchanger.
A first embodiment of the invention is a plate-fin exchanger having a plurality of fins disposed between neighboring parting sheets, at least a portion of at least one of the fins having a textured surface.
A second embodiment is a plate-fin exchanger comprising an assembly of a plurality of substantially parallel parting sheets and a plurality of corrugated fins disposed between adjacent parting sheets, each of the fins having at least one surface, wherein at least a portion of the at least one surface of at least one fin is textured.
A third embodiment is a plate-fin exchanger which includes a first parting sheet and a second parting sheet adjacent and substantially parallel to the first parting sheet. At least one corrugated fin is disposed between the first parting sheet and the second parting sheet, the fin having at least one surface, wherein a surface texture is applied on at least a portion of the surface.
There are several variations of the third embodiment of the plate-fin exchanger. In one variation, at least a portion of the surface texture is in the form of horizontal striations. In another variation, at least a portion of the surface texture is applied at an angle relative to a horizontal position. In a variant of that variation, the angle is greater than about 0xc2x0 degrees and less than about 75xc2x0 degrees. In another variant, the angle is greater than about 0xc2x0 and less than about 50xc2x0.
In another variation, at least a portion of the surface texture is applied in a crisscrossing manner. In yet another variation, the surface texture is in the form of a groove having a wavelength and a range of about 0.5 mm to about 5 mm. In a variant of that variation, the groove is at an angle relative to a horizontal position, the angle being greater than about 0xc2x0 and less than about 75xc2x0.
In another variation, the surface texture is in the form of a groove having a wavelength in a range of about 1 mm to about 3 mm. In yet another variation, the surface texture is in the form of a groove having an amplitude in a range of about 0.05 mm to about 0.75 mm. In a variant of that variation, the groove is at an angle relative to a horizontal position, the angle being greater than about 0xc2x0 and less than about 75xc2x0.
In another variation, the surface texture is in the form of a groove having an amplitude in range of about 0.05 mm to about 0.75 mm. In a variant of that variation, the groove is at an angle relative to a horizontal position, the angle being greater than about 0xc2x0 and less than about 75xc2x0.
In another variation, the surface texture is in the form of a groove having an amplitude in a range of about 0.15 mm to about 0.50 mm. In yet another variation, the surface texture is in the form of a groove having a wavelength in a range of about 0.5 mm to about 5 mm and an amplitude in range of about 0.05 mm to about 0.75 mm. In a variant of that variation, the groove is at an angle relative to a horizontal position, the angle being greater than about 0xc2x0 and less than about 75xc2x0.
Another aspect of the present invention is a cryogenic air separation unit having a plate-fin exchanger as in any of the above described embodiments or variations of those embodiments.
A fourth embodiment of the invention is an improvement to a plate-fin exchanger having at least one corrugated fin disposed between neighboring parting sheets. The improvement is a surface texture applied on at least a portion of the at least one surface.
A fifth embodiment of the invention is a plate-fin heat exchanger for indirect heat exchange of a plurality of fluid streams having a first group of passages adapted to carry a first fluid stream, the first fluid stream being two-phase in at least a portion of the first group of passages, the portion of the first group of passages having a plurality of fins disposed therein, at least one of the fins being disposed between neighboring parting sheets and having a textured surface.
A sixth embodiment is a plate-fin heat exchanger for reboiler or condenser service, the heat exchanger comprising a parallelepipedal body including an assembly of a plurality of substantially parallel parting sheets and a plurality of corrugated fins disposed between adjacent parting sheets, at least one of the fins being disposed between neighboring parting sheets and having a textured surface.
A seventh embodiment is a downflow reboiler having a generally parallelepipedal body formed by an assembly of substantially parallel vertically extending passages adapted to receive a first fluid introduced into a first group of passages and a second fluid introduced into a second group of passages, the passages in the second group of passages alternating in position with the passages in the first group of passages, the first group of passages having a plurality of fins disposed between neighboring parting sheets, the fins including hardway fins for fluid distribution of the first fluid and easyway heat transfer fins downstream of the hardway fins, the heat transfer fins forming one or more heat transfer sections with progressively decreasing surface area, at least one heat transfer fin in a first heat transfer section having at least one surface, the improvement comprising a surface texture applied on at least one surface.
Another aspect of the present invention is a downflow reboiler according to the seventh embodiment installed in a column of an air separation plant wherein a liquid oxygen-containing stream is passed through the first group of passages in parallel flow to a nitrogen-containing and/or argon-containing stream in the second group of passages.
An eighth embodiment of the invention is an improvement to a downflow reboiler having a generally parallelepipedal body formed by an assembly of substantially parallel vertically extending passages adapted to receive a first fluid introduced into a first group of passages and a second fluid introduced into a second group of passages, the passages in the second group of passages alternating in position with the passages in the first group of passages, the second group of passages having a plurality of fins disposed between neighboring parting sheets, the fins including inlet and outlet distribution fins for uniform flow of the second fluid into and out of the second group of passages and heat transfer fins forming at least one heat transfer section between the inlet and outlet distribution fins, at least one heat transfer fin in the at least one heat transfer section having at least one surface, the improvement comprising a surface texture applied on the at least one surface.
Another aspect of the invention is a downflow reboiler according to the eighth embodiment installed in a column of an air separation plant wherein a liquid oxygen-containing stream is passed through the first group of passages in parallel flow to a nitrogen-containing and/or argon-containing stream in a second group of passages.
A ninth embodiment is a plate-fin exchanger for dephlegmator service, the exchanger comprising a parallelepipedal body including an assembly of a plurality of substantially parallel parting sheets and a plurality of corrugated fins disposed between adjacent parting sheets, at least one of said fins being disposed between neighboring parting sheets and having a textured surface.
The present invention also includes a method for assembling a plate-fin exchanger. The method includes multiple steps. The first step is to provide two substantially parallel parting sheets and an elongated sheet. The second step is to form a surface texture on the elongated sheet. The third step is to corrugate the elongated sheet to form a fin having the surface texture thereon. The fourth step is to dispose the fin having the surface texture thereon between the parting sheets.
In a variation of the method for assembling a plate-fin exchanger, at least a portion of the surface texture is in the form of at least one groove having a wavelength in a range of about 0.5 mm to about 5 mm and an amplitude in a range of about 0.05 mm to about 0.75 mm, the at least one groove being at an angle relative to a horizontal position, the angle being greater than about 0xc2x0 and less than about 75xc2x0.
The present invention also includes a method for improving the performance of a plate-fin exchanger having at least one fin between neighboring parting sheets, comprising applying a surface texture on at least a portion of the at least one fin.